1. Field of the Invention
The present invention relates to amplifiers.
2. Prior Art
This invention applies to an amplifier class with a high impedance output that sources current to an external load resistor to set the amplifier output voltage. It is applicable to but not limited to precision amplifiers where high gain accuracy and low offset are important. It is also useful in multiplexed output applications in which a single external load resistor is shared by many amplifier outputs where it is convenient for the amplifiers to be powered at various supply voltages or to have some of the multiplexed amplifiers powered down.
There is a first set of applications where it is useful for the amplifier to have a bi-directional output where the zero value is equal to an external reference voltage, Vref, near Vcc/2 (supply voltage divided by 2). In these applications the external load resistor is tied to Vref. The most positive output is near Vcc, and the most negative output is near G (ground). There is a second set of applications that are unidirectional where the zero output is at G, exactly. In these applications the load resistor is tied to G, and the most positive output is near Vcc. These two sets of applications place different requirements on the output structure of the amplifier and, as will be described later, are not generally possible in a single, monolithic amplifier IC.
An incentive for having an external load resistor in precision applications is to remove the error that would otherwise be introduced due to minor ground shifts between the amplifier and the device sensing the amplifier output. In many of these applications there may be a great distance between the two ICs and positive and negative ground shifts can be significant relative to the error budget of the system. In these applications, when G is used as the reference voltage, it is necessary for the output to be able to go below local IC ground.
Another set of problems particular to amplifiers with unidirectional outputs is maintaining constant gain while approaching zero output, and achieving an absolute zero output, which requires being able to completely turn off the current going to the external load resistor.
An additional incentive for having an external load resistor is that multiple amplifiers may share a single load resistor in a multiplexed arrangement where a single amplifier is enabled while all others are disabled. This is useful in an application where the outputs of many amplifiers are read by a single Analog to Digital Converter (ADC). A problem in this case is that all amplifiers in the multiplexed system must always remain powered at the same supply voltage. If a single amplifier were powered down (power supply terminal is below normal system supply voltage or near G) the multiplexed output would be shorted through the body diode of the output driver of the powered down amplifier. While it is common in industrial communication and interface products to protect against this problem, it is not common in precision amplifiers. U.S. Pat. No. 5,414,314 by Thurber, Jr. and assigned to the assignee of the present invention covers a high-side (PMOS) connection and a low-side (NMOS) connection, both of which provide a means for isolating an output from the supply and from ground. U.S. Pat. No. 5,963,067 by Boucher and assigned to the assignee of the present invention provides a means for active rectification and makes use of the connection covered by Thurber, Jr.